The present invention relates generally to a gas diffusion electrode assembly and a bonding method for gas diffusion electrodes, and more particularly to a bonding method for gas diffusion electrodes to build up a gas diffusion electrode assembly having a large electrode area and an electrolyzer comprising gas diffusion electrodes.
Aqueous solutions are electrolyzed to produce various materials while hydrogen is generated at cathodes in general. Hydrogen generated by electrolysis is higher in purity than that obtained by other means. In applications where their main purpose is not to obtain hydrogen, however, it is proposed to avoid generation of hydrogen to lower electrolysis voltage, thereby driving down power per electrolytic reaction.
That is, when oxygen gas diffusion electrodes are used as cathodes in place of general hydrogen generation electrodes, it is possible to achieve a 1.2 V drop of electrolysis voltage because hydrogen is not generated at the cathodes. With the gas diffusion electrodes, accordingly, the electrical energy necessary for electrolysis can be saved. Thus, electrolysis harnessing oxygen gas diffusion electrodes as cathodes is extremely effective at plants with no condition for efficient use of hydrogen.
To prevent generation of hydrogen at cathodes thereby stepping down electrolysis voltage, there has been proposed an electrolysis process where oxygen is fed to an oxygen gas diffusion electrode placed at a cathode.
Creating a gas-liquid-solid three-phase interface at a reaction layer site, a gas diffusion electrode is now produced by forming a composition comprising a hydrophobic synthetic resin such as fluororesin, a catalyst, an electrically conductive substance, etc., or sintering the resultant formed product.
On the other hand, industrially available aqueous solution electrolyzers represented by ion-exchange membrane electrolyzers for brine are becoming larger and larger, and with this an electrode area of as large as a few m2 is being employed.
On such a large electrolyzer a gas diffusion electrode having a large area must be mounted. However, it is cumbersome to produce a large-area gas diffusion electrode. Made of a composition comprising fluororesins, electrically conductive substances, etc., a gas diffusion electrode is of low mechanical strength, and as area increases, it tends to undergo deformation due to its own weight, and is difficult to work with as well.
To produce an electrolyzer having a large electrode area, a number of gas diffusion electrodes, each having a small area, must be provided. In this case, they must be bonded together in such a way as to ensure prevention of leakage of electrolytes and gases through junctions at which they are bonded.
However, gas diffusion electrodes are made of a composition comprising an electrically conductive material such as carbon black and a fluororesin, and so are just only poor in adhesion to other member but lacking in strength as well. It is thus impossible to bond or fix them together with large force, offering a problem with reliable bonding.
For instance, any sufficient bonding strength could never be obtained by use of fluororesins such as polytetrafluoroethylene. On the other hand, tetrafluoroethylene-hexafluoropropylene copolymers (FEPs) are known as fluororesins having much the same heat fusion bonding properties as general-purpose thermoplastic resins. Gas diffusion electrodes may be bonded together by heat fusion bonding of an EFT film placed on a junction between them. However, even when a thin FEP film is used, the heat fusion bonded junction often warps up in a concave form toward the FEP side. Such an irregular bonded surface may induce stress in the vicinity of the junction and do damage thereto, or render it difficult to mount the gas diffusion electrodes on an electrolyzer.
Projections from the warping junction may do damage to ion-exchange membrane surfaces.
To produce an electrolyzer having a large electrode area, a number of gas diffusion electrodes, each having a small area, must be provided as described above. In this case, they must be bonded together in such a way as to ensure prevention of leakage of electrolytes and gases through junctions at which they are bonded together.
For instance, JP(A) 2000239881 proposes an electrolyzer comprising a number of gas diffusion electrodes, wherein they are bonded by hot pressing to two or more openings provided in a corrosion-resistant metal frame via a silver sheet for the purpose of ensuring prevention of electrolytes and gases.
However, a problem with hot pressing of fluoro-resin-containing gas diffusion electrodes is that any sufficient bonding properties are not obtained at their junctions with a metal sheet.
Currents are conducted through a gas diffusion electrode by connecting a portion (that serves as an electrically conductive connector of the gas diffusion electrode) of a collector drawn out of its gas supply layer in which the collector is embedded with a cathode pan that also serves as a cathode chamber collector, so that the collector can be electrically connected to the cathode pan.
A junction of the collector in the gas diffusion electrode with the cathode pan must be of low electrical resistance, and fully sealed up in such a way as to prevent leakage of electrolyte, etc. through that junction.
To an industrially used electrolyzer of as large as 1 m×1 m or more, a plurality of gas diffusion electrode units having a width of, for instance, about 30 cm are often bonded and attached in a divided fashion in consideration of difficulty with which a large-area gas diffusion electrode is produced, electrically conductive connection with a cathode pan or the like. Even such gas diffusion electrode units are bonded to the electrolyzer, the junctions must be of reduced electrical resistance, and be sealed up in such a way as to prevent leakage of electrolytes.
For instance, JP(A) 2000119880 comes up with an attachment method for such gas diffusion electrodes, wherein electrical conductors mounted on the gas diffusion electrodes are inserted in a groove in a cathode collector frame with wedges embedded therein, thereby bringing the cathode collector frame in contact with the gas diffusion electrodes.
JP(A) 2000199094 proposes a method for prevention of penetration of electrolytes, wherein collectors exposed on the peripheries of gas diffusion electrodes are welded to a cathode collector frame at junctions, and the junctions are sealed up with a sealing agent.
JP(A) 2000273679 puts forward a sealing method for gas diffusion electrodes wherein polytetrafluoroethylene (PTFE) fine powders are filled in junctions of the gas diffusion electrodes and the junctions are sealed up by ultrasonic fusion bonding, or a bonding method for gas diffusion electrodes wherein the gas diffusion electrodes are bonded together using as an adhesive a solution of polyethersulfone resin in an organic solvent.
JP(A) 2000239879 proposes a method wherein a cathode pan is bonded to resin sheets hot-pressed to gas diffusion electrodes.
One object of the present invention is to provide a bonding method for gas diffusion electrodes. Another object of the invention is to provide a gas diffusion electrode assembly that can be attached to an electrolyzer having a large electrode area with junctions having improved airtightness.
Yet another object of the invention is to provide an electrolyzer having a large electrode area. A further object of the invention is to provide an electrolyzer having a large electrode area, which, upon assembled, is less likely to suffer from leakage of electrolytes and gases through joints of a plurality of gas diffusion electrodes mounted on an electrolytic surface.
A further object of the invention is to provide a sealing method for gas diffusion electrodes, which ensures reliable prevention of liquid leakage and in which junctions of the gas diffusion electrodes are of low electrical resistance, and the junctions can not only be used as mere junction areas but also can be effectively used as electrodes.